In the manufacture of refined sugar from either sugar cane or sugar beets, it is important to control the process so as to yield as much sugar as possible from the sugar bearing plant. An advantage derived from the increased yield is the lower cost for the manufacture of refined sugar. The more sugar derived from the sugar plant, the less expensive it is to manufacture the sugar.
The production of cane sugar for human consumption generally comprises two distinct operations, namely the production of raw sugar and the production of refined sugar, which are often carried out in separate locations.
Production of raw sugar from sugar cane typically takes place at a sugar mill, which is usually located in or near sugar cane fields. In the mill, sugar cane stalks are chopped into pieces and the pieces are crushed in a series of roller mills in order to remove the juice. The juice from the first set of roller mills is referred to as “first juice,” while the total juice from all the roller mills in the process is referred to as “mixed juice.” The juice is normally limed, deaerated and clarified (i.e., removal of suspended solids, usually by sedimentation). The clarified stream is referred to as “clarified juice.” The juice is then evaporated in a series of evaporators to produce a concentrated juice, that is, a thick syrup (known as “evaporated juice”).
Sugar syrups, for the present purpose, are sugar solutions which are in the process of being refined. A molasses is syrup resulting from a refining process that contains some or most of the impurities of the original syrup. For identification, a syrup molasses is one resulting from the separation of sugar crystals from what has previously been a concentrated sugar-water mixture.
Sugarcane syrup from the evaporators is sent to vacuum pans, where it is further evaporated, under vacuum, to supersaturation. Fine seed crystals can be added, and the sugar (“mother liquor”) yields a solid precipitate of about 50% by weight crystalline sugar. Crystallization can be a serial process. The first crystallization, also known as a strike and in this instance as an A strike, yields massecuite A that is separated into sugar A and molasses A. The term massecuite is defined as the mixture of sugar liquor and sugar crystals that is formed in the vacuum pans. The molasses A is concentrated to yield a B strike, which becomes massecuite B. The obtained massecuite B is separated into sugar B and a low-grade molasses B. The molasses B can be concentrated to yield a C strike, and the obtained massecuite C can be separated to yield a sugar C and final molasses C, sometimes referred to as blackstrap molasses. The sugar C is used as a footing in the A and B strikes. The separated sugar A and sugar B are combined and dried to produce raw sugar.
Depending on the exact nature of the process steps and conditions used in the sugar mill, the raw sugar product can be made purer. A more highly purified mill product is sometimes referred to as “Mill White”, “Plantation White”, or “Crystal” sugar. The production of these sugars requires sulfitation to bleach the sugar, before or after clarification, using SO2 gas. It usually requires a second clarification step, usually at the syrup stage and sometimes a second sulfitation step. In nearly all cases the ash content of this sugar is much higher, perhaps by more than four times, that of refined white sugar. Although these particular mill products can be sold for human consumption without further processing in some instances, generally raw sugar must be further refined before it reaches a commercially acceptable level of purity, particularly for subsequent use by food and drink manufacturers.
Therefore, the raw sugar from a mill is usually transported to a sugar refinery for further processing. In a conventional cane sugar refining process, the raw sugar is subjected to affination by first washing the raw sugar with a not quite saturated solution of sucrose and centrifuged to remove the molasses film from the outside of the raw sugar crystals. The sugar obtained upon centrifugation is called washed sugar. The syrup formed is called affination syrup. The washed sugar is melted (dissolved) in hot water and the pH is adjusted with lime to form melter liquor.
The melter liquor is then purified, generally by the successive steps of clarification and decolorization. The clarification step usually involves forming an inorganic precipitate in the liquor, and removing the precipitate and along with it insoluble and colloidal impurities which were present in the melter liquor. In one of the clarification processes commonly used for melter liquor, termed “phosphatation,” the inorganic precipitate is calcium phosphate, normally formed by the addition of lime and phosphoric acid to the liquor. The calcium phosphate precipitate is usually removed from the liquor by flotation, in association with air bubbles. Other clarification processes, termed carbonation (or carbonatation) processes, involve adding lime and carbon dioxide to the liquor, and produce calcium carbonate precipitate.
The resulting “fine liquor” is boiled to white sugar in a series of vacuum pans similar to those used in sugarcane processing. The boiling system can comprise at most six or seven stages of boiling. The first three or four strikes can be blended to make commercial white sugar. Special large-grain sugar for bakery and confectionery can be boiled separately. Fine grains or fruit sugars can be produced by sieving products of mixed grain size. Powdered icing sugar, or confectioners' sugar, results when white granulated sugar is finely ground, sieved, and mixed with small quantities of starch or calcium phosphate to keep it dry. Brown sugars (light to dark) can be either crystallized from a mixture of brown and yellow syrups with caramel added for darkest color or made by coating white crystals with a brown-sugar syrup.
Although similar in principle, the process of purifying beet sugar juice differs considerably from that employed in the raw cane sugar process. The production process of raw sugar from sugar beets starts by slicing the beets into thin chips or strips called cossettes. The sucrose is extracted from the cossettes by passing them through a diffuser countercurrent to a flow of hot leaching water. At one end of the equipment, the cossettes enter and the raw juice leaves. At the other end, the exhausted cossette pulp leaves and the hot water enters. The pH of the feed water is controlled in order to reduce the formation of invert sugar during diffusion.
In beet sugar factories, lime and carbon dioxide are used for juice purification. On passing carbon dioxide through a limed juice mixture, either simultaneously with or after liming, calcium carbonate is precipitated. The calcium carbonate crystals with their adsorbed impurities tend to cluster around pieces of organic matter, which are so large that they settle readily in tray clarifiers and can be removed satisfactorily from the underflow by vacuum filters to produce a thin juice.
The thin juice is subjected to sulfitation by the addition of a small quantity of sulfur dioxide. The sulfur dioxide is not used for bleaching the juice, but to catalytically inhibit the Maillard or browning reaction between reducing sugars and amino acids. This browning reaction is the chief cause of increased coloration of the process juices during evaporation and crystallization.
After sulfitation, the thin juice is then sent to a series of evaporators to remove moisture in order to produce a thick juice. The thick juice is subjected to vacuum pan boiling which produces a strike called massecuite. The massecuite comprises sugar crystals in a syrup of molasses. The massecuite is separated such that raw sugar crystals are removed for storage, with the remainder being the molasses. This molasses is sent to a second vacuum pan to produce a second strike massecuite of sugar crystals in a syrup. The sugar crystals are removed from this massecuite, sent to storage to be combined with the previously obtained sugar crystals. The combined sugar crystals are dried, screened, cooled, and bagged or stored in bulk.
Beet sugar processing equipment such as filters, vacuum pans, centrifuges, granulators, dryers, and screens is similar to that employed in cane refineries. Beet sugar products, as manufactured in the United States, are essentially identical with refined cane sugar products.